The invention relates generally to telephone handsets containing receiver amplifiers for use by the hearing-impaired or for use in environments with high ambient noise levels.
Conventional telephone handsets have no means for varying the amplitude of the audio signal received from the telephone subset. Thus, the volume of the acoustic output of the receiver is not adjustable, and is preset at a level sufficient for persons with normal hearing. This poses an obvious disadvantage for persons having hearing impairments. It is also disadvantageous where the telephone is located in an environment with high ambient noise levels, such as manufacturing facilities.
One solution to this problem is to provide a telephone handset with a receiver amplifier and a volume control in the form of a potentiometer. The potentiometer allows the user to vary the receiver "volume" to suit the user's need. A receiver amplifier of this type is disclosed in U.S. Pat. No. 4,151,376. Other telephone handset amplifiers are also well known. For example, see U.S. Pat. Nos. 3,254,160 and 3,830,979.
Conventional telephone handsets almost universally employ the variable-resistance granular carbon-type transmitter. This type of transmitter responds poorly to low sound levels and has limited frequency response and appreciable distortion.